The present disclosure relates to a control device of a motor configured to apply PWM (pulse width modulation) control to switching elements inside a bridge circuit that form a current path to the motor, and an electric power tool provided with the control device.
The control device comprises a bridge circuit having a plurality of switching elements (a plurality of high-side switches and a plurality of low-side switches) between a plurality of terminals of the motor and positive and negative electrodes of a direct current (DC) power source.
The control device, when the motor is driven, selects a high-side switch and a low-side switch to be use to form a current path, depending on a rotation position of the motor, and holds one of the switches in an ON state and alternately turns on/off the other by a PWM signal having a predetermined duty ratio. Thereby, electric current flowing to the motor is PWM controlled.
Also known is a PWM control method which not only turns on/off a switching element on a current path from a DC power source to a motor by a PWM signal, but turns on/off a switching element connected to the same motor terminal as that switching element in a reverse phase.
This control method is called complementary PWM, which is, for example, disclosed in Japanese Unexamined Patent Application Publication No. 2009-261223. The complementary PWM can suppress temperature rise of a bridge circuit, as compared to non-complementary PWM which turns on/off one switching element by a PWM signal.
Diodes are connected in parallel to the switching elements inside the bridge circuit. The diodes corresponding to the respective switching elements are used to flow electric current deriving from energy stored in motor winding from a negative electrode toward a positive electrode of the DC power source, when the current path to the motor is interrupted by other switching elements.
In the non-complementary PWM, when the switching element to be turned on/off by the PWM signal is turned off, electric current flows through a diode connected to the same motor terminal as that switching element, so that electric current continues to flow to the motor winding.
When electric current flows to the diode as such, the diode is heated by internal resistance. Temperature rise occurs in the switching element (and the bridge circuit) provided with the diode.
In contrast, in the complementary PWM, when the switching element on the current path from the DC power source to the motor is turned off by a PWM signal, a switching element connected to the same motor terminal as that switching element is turned on, and electric current flows to the turned-on switching element.
As a result, according to the complementary PWM, electric current flowing through the diode is suppressed, temperature rise of the switching element (and the bridge circuit) caused by heat generation of the diode is suppressed.